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calibration of contact force and deflection sensitivity for AFM indentation tests

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Aaron posted on Wed, Sep 18 2013 6:28 AM

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Hello Bruker, hello users,

I am doing AFM indentation and came across some problems concerning calibrations.

Through the exchange with Bruker I learned that I have to calibrate the deflection sensitivity and the contact area. The user manual tells me that I have to calibrate the deflection sensitivity by indenting sapphire. After I did it I found out that the calibrated deflection sensitivity leads to such curves (in this case Au):

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After I asked Bruker I was told that I have to tune the deflection sensitivity manually afterwards. I guess instead of doing it manually I should indent a material with known properties in order to calibrate it carefully.

To calibrate the contact area I was told I have to indent fused silica with a known E-modulus at different indent depth. First I thought the specs given on Brukers webpage for the cantilever would be sufficient. It is obviously not the case.

So far so good. However, now I have a problem. I have to calibrate the deflection sensitivity and the contact area basically at the same time. How is this possible? How can I calibrate two thing with one measurement?

Further, do you have any ideas where the spikes in the curves come from? It  seems to be systematic and not material inherent.

Thank you for your help.

Aaron


 

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Aaron replied on Wed, Sep 18 2013 6:32 AM

sorry for the mess in my message: the image got lost...

I try it again:

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Dear Aaron,

I guess your second trial with the images was not more successful thatn the first one. Try to follow these tips: http://nanoscaleworld.bruker-axs.com/nanoscaleworld/forums/t/526.aspx

Regarding your questions: It would be interesting to know which probes you use (spring constant etc.). The deflection sensitivity must be calibrated on an "infinitely" stiff material. It shouldn't have "known properties" but should have a modulus way stiffer than the one you want to measure. You want your reference to not deform at all so that your total deflection equals the deflection of your probe.

Regards, Dietmar

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Aaron replied on Thu, Sep 19 2013 3:03 AM

Dear Dietmar,

thank you for the hint. I guess it is better now:

Coming back to your comments. I used the following:

Dimension ICON in combination with Nanoscope V controller and PDNISP cantilever. Calibration of the deflection sensitivity was done with Saphire, as described in the manual. Still, Bruker told me to tune the deflection sensitivity afterwards.

Thank you for your help

Aaron

 

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I was told something similar. However, the deflection sensitivity on the saphire is a good first step. If you want to tune it afterwards you must compare at least two different materials of known modulus and adjust the deflection sensitivity in a way that the ratio between the measured moduli is correct. Let's say you measure ramps on one reference material that should have a modulus of 30 GPa and on another one that should have 3 GPa. Then you can adjust the deflection sensitivity (offline) until the ratio of the measured moduli is ten. Then your deflection sensitivity is correct.

I have no experience with nanoindentation so far, but I think the process is similar. I also cannot tell you if your curves are fine or not, but they do look strange to me.

Dietmar

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Aaron replied on Mon, Sep 23 2013 4:45 AM

Thank you Dietmar for your help. We'll see what I can do. But ok- your suggestion with 2 or more material sounds reasonable.

Aaron

 

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Hi Aaron,

if those discontinuities in your force curve are what I think they are, try turning closed loop off before you do your measurement to get rid of them.

Cheers,

Ash

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Aaron replied on Mon, Oct 28 2013 5:13 AM

Hi Ash,

we will give it a try and see what it changes. Thank you very much.

Aaron

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Igor replied on Fri, Nov 1 2013 10:07 AM

Hi Aaron,

I feel there is a mix of two techniques in your questions: AFM indentation and nanoindenter indentation. AFM indentation works well only for relatively soft materials (unless you use very special cantilevers, stiff,  very long, with diamond tip, follow special calibration and may still need to use a special mode (lateral correction of the sample during indentation, which may be commercially unavailable)).

So question #1: which materials do you try to indent? I suspect from your curves that rather hard ones..

--Igor

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Bruker Employee

Hi Aaron,

It looks as if you have found a bug.  The issue is caused by a slight lateral motion that helps improve the scanner orthogonality in imaging.  This is redundant in ramp mode because we have X-rotate which does approximately the same thing, but with a different DAC.  We are working on a fix for the software, but in the meantime, you should be able to solve the problem by setting the "Z to X coupling" to zero (look in the Calibrate>Scanner>Z dialog).

Thanks for reporting this.

--Bede

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Aaron replied on Wed, Nov 6 2013 2:00 PM

Hi Igor,

as far as I understood it, there should not be a difference between AFM and Nanoidentation. Of course each method has its errors- and AFM has more of them, that's why Nanoindentation is preferred. However we tried to indent small wires, which was not possible with the Nanoindenters available here.

Anyway, the material is Ag. The curve was acquired not from Ag but from a reference (Sapphir).

Aaron

 

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Aaron replied on Wed, Nov 6 2013 2:04 PM

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Hello Bede,

Thank you for your info. I appreciate it if you can send me the bug fix.

Everything that was posted so far sounds like I can forget my original measurements. Too bad- it took over a week to prepare the samples and characterize them.

I will have time to test all suggestions and hints in the coming week.

Aaron

 

 

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Aaron replied on Thu, Nov 21 2013 10:06 AM

We checked some parameters. What we found is that the z-closed loop does not causes the spikes but the x-y-cloosed loop. However we need the x-y-closed loop if we would like to preciclely indent at a certain position.
The overshoot or loop when withdrawing the indenter seems to come from the adhesion of the cantilever on the sample. When we indent less deep/ with less force we do not see the loop any more/or a lot less.

Aaron

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Please see my comment above about the Z-X coupling.  You should be able to use CLXY if you set the Z-X coupling to zero.  

The reason the spikes go away when CLXY is off is because the Z-X coupling is disabled when CLXY is off.

--Bede

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